receptor for acetylcholine; primarily found on organs innervated by the parasympathetic nervous system; include M1 and M2 receptors; binding generally leads to smooth muscle contraction

adrenergic receptors

G-protein coupled receptors that are the targets for catecholemines; primarily found on organs innervated by the sympathetic nervous system; include alpha-1 and beta-1 and beta-2 receptors; binding generally leads to smooth muscle relaxation

enteric nervous system

intrinsic to the gastrointestinal tract; not directly controlled by the CNS; modulated by ANS modulation (primarily parasympathetic)